1. Field of the Invention
The present invention relates to a base band processor capable of being applied to OFDM-based communications systems and suppressing peak power of a transmission signal by using a variable sampling rate and window clipping, a transmitter with the same, and a method of processing a transmitting signal.
2. Description of the Related Art
Generally, one of the biggest defects of communication modulation applied to orthogonal frequency-division multiplexing (OFDM)-based communications systems, such as long term evolution (LTE), worldwide interoperability for microwave access (WiMax), and the like, is high peak to average power ratio (PAPR or PAR). The reason for this is that efficiency of a linear power amplifier may be degraded due to a high PAPR (or peak). Therefore, many PAPR suppression (or peak suppression) methods have been researched and developed over the last decades.
As a method, among existing PAPR suppression methods, that can be implemented most efficiently and easily, there is provided a method of window clipping. The PAPR suppression method can be applied to the OFDM-based communications systems without changing any protocol or a process of generating a standard signal.
However, in the latest OFDM-based communications systems, such as OFDMA, Single_Carrier_Frequency_Division_Multiple Access (SC-FDMA), and the like, a signal bandwidth may be changed in real time according to use environment. Therefore, the thickness of a typical peak signal may be seriously changed. However, when the window size is fixed as in the related art, the PAPR may not be efficiently suppressed.
In addition, when a bandwidth of a communication signal is wide, an original sampling rate of a signal may not be enough to measure the information of the peak signal in the OFDM-based communications systems. In this case, the efficiency of the window clipping method may be more seriously degraded.
On the other hand, when the bandwidth of the communication signal is narrow, the original sampling rate may be higher than needed. In this case, the sampling rate is to be reduced so as to reduce complexity of the window clipping without degrading performance.
Therefore, when the bandwidth of the communication signal is seriously changed in real time, problems of window size, inaccurate information detection of a peak signal, and complexity may occur.
Related Art Document 1 discloses “Low splatter peak-to-average signal reduction”, which discloses that PAPR can be effectively suppressed while reducing spectrum distortion that is a general problem of -PAPR suppression. However, when the thickness of the peak signal is thin, the peak signal cannot be accurately measured due to the insufficient sampling rate and thus, the performance may be degraded.
Related Art Document 2 relates to “Low-splatter peak-to-average signal reduction with interpolation”, which discloses a technical matter of interpolating a signal with peak and then, applying original window clipping, when the sampling rate is not sufficiently large. Therefore, the method can solve a problem that the sampling rate is not sufficient. However, in the OFDMA-based communications systems in which the bandwidth is changed in real time, such as in LTE, the problems of the window size or the too large or too small sampling rate cannot be solved in real time.